Method for producing cyclic lactams

ABSTRACT

A process is provided for the preparation of cyclic lactams of formula (II):  
                 
 
     in which n and m can each have the values 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 and the sum of n+m is at least 3, preferably at least 4, and R 1  and R 2  are C 1 -C 6 -alkyl, C 5 -C 7 -cycloalkyl or C 6 -C 12 -aryl groups,  
     by reacting a compound (I) of the formula  
                 
 
     in which R 1 , R 2 , m and n are as defined above and R are [sic] nitrile, carboxamide and carboxylic acid groups,  
     with steam in the gas phase, wherein  
     a) the compound (I) is reacted with steam in the gas phase with the addition, before or after the reaction, of an organic diluent (III) which exhibits a miscibility gap with water under specific quantity, pressure and temperature conditions,  
     to give a mixture (IV) containing a lactam (II),  
     b) the mixture (IV) is converted, before or after the separation of ammonia, under quantity, pressure and temperature conditions such that the diluent (III) and water are liquid and exhibit a miscibility gap, to give a two-phase system consisting of a phase (V) containing a higher proportion of diluent (III) than water, and a phase (VI) containing a higher proportion of water than diluent (III),  
     c) the phase (V) is separated from the phase (VI), and  
     d) the diluent (III) and optionally by-products selected from the group consisting of low-boiling component [sic], high-boiling component [sic] and unreacted compound (I) are separated from the phase (V) to give a lactam (II).

[0001] The present invention relates to a process for the preparation ofcyclic lactams of formula (II):

[0002] in which n and m can each have the values 0, 1, 2, 3, 4, 5, 6, 7,8 and 9 and the sum of n+m is at least 3, preferably at least 4, and R¹and R² are C₁-C₆-alkyl, C₅-C₇-cycloalkyl or C₆-C₁₂-aryl groups,

[0003] by reacting a compound (I) of the formula

[0004] in which R¹, R², m and n are as defined above and R are [sic]nitrile, carboxamide and carboxylic acid groups,

[0005] with steam in the gas phase, wherein

[0006] a) the compound (I) is reacted with steam in the gas phase withthe addition, before or after the reaction, of an organic diluent (III)which exhibits a miscibility gap with water under specific quantity,pressure and temperature conditions,

[0007] to give a mixture (IV) containing a lactam (II),

[0008] b) the mixture (IV) is converted, before or after the separationof ammonia, under quantity, pressure and temperature conditions suchthat the diluent (III) and water are liquid and exhibit a miscibilitygap, to give a two-phase system consisting of a phase (V) containing ahigher proportion of diluent (III) than water, and a phase (VI)containing a higher proportion of water than diluent (III),

[0009] c) the phase (V) is separated from the phase (VI), and

[0010] d) the diluent (III) and optionally by-products selected from thegroup consisting of low-boiling component [sic], high-boiling component[sic] and unreacted compound (I) are separated from the phase (V) togive a lactam (II).

[0011] Processes for the preparation of cyclic lactams by reactingomega-aminocarboxylic acid derivatives with steam in the gas phase inthe presence of heterogeneous catalysts, for example the preparation ofcaprolactam from 6-aminocarboxylic [sic] acid nitrile, are generallyknown.

[0012] Thus WO 96/22974, EP-A-659741, WO 99/47500 and WO 99/28296disclose the reaction of 6-aminocapronitrile with steam in the gas phasein the presence of heterogeneous catalysts to give caprolactam andammonia, examples of the heterogeneous catalysts used being aluminumoxide, lanthanum phosphates and zirconium dioxide. WO 96/22974 pointsout that it is not excluded to carry out the cyclization using a diluentwhich is inert under the reaction conditions, for example an alkane, acycloalkane, an aromatic hydrocarbon or a halogenohydrocarbon, and thusto have a liquid phase in the reaction mixture.

[0013] WO 98/05636 describes the work-up of reaction discharges such asthose obtained in the gas phase cyclization of 6-aminocapronitrile withsteam in the presence of solid catalysts. To effect said work-up:

[0014] a) the bulk of the ammonia formed in the cyclization is separatedfrom the water-containing crude caprolactam, and

[0015] b) this crude caprolactam is either subjected to a liquid/liquidextraction with the addition of a solvent containing acidic groups,and/or treated with a cation exchanger.

[0016] Measure b) extracts all the amines or binds them to the cationexchanger. The principal disadvantage of this work-up is thataminocapronitrile which has not reacted in the cyclization is taken upby the ion exchangers and, when the ion exchanger is regenerated, isobtained as the ammonium salt mixed with ammonium salts of amineby-products, so it cannot be returned to the cyclization. Ammonium saltsare also formed in the extraction of the aminocapronitrile withextractants containing acidic groups, and create the same problems.

[0017] It is therefore an object of the present invention to provide aprocess which makes it possible to prepare cyclic lactams (II) fromcompounds (I) in a technically simple and economic manner, produces highyields of lactam with high conversions of the compound (I) and alsominimizes losses of yield in the work-up.

[0018] We have found that this object is achieved by the process definedat the outset.

[0019] Suitable compounds (I) are aminocarboxylic acids and derivativesthereof, preferably those of general formula I:

[0020] in which R is carboxylic acid, nitrile and/or carboxamide groupsand n and m can each have the values 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 andthe sum of n+m is at least 3, preferably at least 4.

[0021] In principle, R¹ and R² can be any kind of substituents, the onlyproviso being that the desired cyclization reaction is not affected bythe substituents. Preferably, R¹ and R² independently of one another areC₁-C₆-alkyl or C₅-C₇-cycloalkyl groups or C₆-C₁₂-aryl groups.

[0022] Particularly preferred starting compounds are aminocarboxylicacid nitrites, preferably those of the general formula

H₂N—(CH₂)_(m)—C≡N

[0023] in which m has a value of 3, 4, 5 or 6, especially 5. When m=5,the starting compound is 6-aminocapronitrile.

[0024] The compound (I) used can be a single compound (I) or a mixtureof different compounds (I). The compound (I) used is preferably a singlecompound.

[0025] Omega-aminocarboxylic acid nitrites are obtainable for example bythe partial hydrogenation of alpha,omega-dinitriles in the gas or liquidphase, e.g. according to WO 96/20166, WO 96/20916 or WO 96/20165.

[0026] Omega-aminocarboxylic acids are obtainable for example by theaminating hydrogenation of omega-formylcarboxylic acids or by thehydrolysis of omega-aminocarboxylic acid esters or omega-aminocarboxylicacid nitriles.

[0027] Omega-aminocarboxamides are obtainable for example by thereaction of omega-aminocarboxylic acids and esters thereof with ammoniaor by the reaction of omega-aminocarboxylic acid nitrites with water.

[0028] The process according to the invention yields the cyclic lactamsof formula (II) corresponding to the compound (I):

[0029] in which n, m, R¹ and R² are as defined above. Particularlypreferred lactams are those in which n=0 and m has a value of 3, 4, 5 or6. When m=5, the product obtained is caprolactam.

[0030] In step a) of the process according to the invention, theabove-described compound (I) is reacted with steam in the gas phase,advantageously in the presence of a heterogeneous catalyst andoptionally in the presence of an organic diluent (III), to give amixture (IV) containing a lactam (II).

[0031] Suitable heterogeneous catalysts are any of the catalystsdescribed for the gas phase cyclization of a compound (I) to a lactam(II), such as amorphous titanium dioxide in the form of anatase orrutile, aluminum oxide, lanthanum phosphates, hafnium oxide, zirconiumdioxide or mixtures thereof, preferably titanium dioxide, aluminumoxide, lanthanum phosphates, zirconium dioxide or mixtures thereof.

[0032] The diluent (III) can be added before the cyclization. In thiscase the diluent (III) is present during the cyclization of the compound(I) in the gaseous reaction mixture, preferably in amounts of 0.1 to 20g of diluent (III) per gram of compound (I).

[0033] It is preferred to add the diluent (III) to the cyclizationdischarge, for example by quenching the cyclization discharge.

[0034] A further possibility is to add the diluent (III) after theseparation of ammonia from the mixture (IV).

[0035] The diluents (III) used can be organic compounds which exhibit amiscibility gap with water under specific quantity, pressure andtemperature conditions, especially below the reaction temperature in thecyclization.

[0036] Suitable diluents (III) are C₄- to C₉-alkanols such as n-butanol,i-butanol or n-pentanol, preferably aliphatic hydrocarbons such asn-hexane, cycloaliphatic hydrocarbons such as cyclopentane orcyclohexane, and especially aromatic hydrocarbons such as benzene,toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, i-propylbenzene ordi-i-propylbenzene, and mixtures of such compounds, for examplepetroleum ethers. The hydrocarbons can carry functional groups such ashalogens, for example chlorine, as in chlorobenzene.

[0037] Ammonia can also be present in the reaction of step a).

[0038] The reaction can advantageously be carried out in the gas phaseat temperatures generally of 200 to 450° C., preferably of 250 to 400°C. The pressure should generally range from 0.01 to 20 bar, preferablyfrom 0.1 to 5 bar. The reaction mixture should be predominantly gaseousin this reaction. The residence times generally range from 0.1 to 100seconds, preferably from 1 to 50 seconds.

[0039] The amount of water used in the cyclization should generally beat least 0.5 mol, preferably at least 1 to 50 mol, per mol of compound(I).

[0040] The reaction can be carried out in the presence of an inert gassuch as nitrogen or a noble gas or mixtures of such gases.

[0041] The reaction of step a) gives a mixture (IV) containing a lactam(II).

[0042] Provided the mixture (IV) contains ammonia, the ammonia can beseparated from the mixture (IV) in step b) after or, preferably, beforethe phase separation, preferably by distillation, to give anammonia-free or ammonia-poor mixture (IX). The ammonia can also beseparated from the phase (V) and/or the phase (VI) after the phaseseparation, preferably by distillation.

[0043] The mixture (IV) can contain ammonia e.g. if ammonia is formed inthe reaction of step a) and/or if ammonia has been added to the reactionmixture in the reaction of step a). Ammonia can be formed in thereaction of step a) e.g. if R is a nitrile or carboxamide group.

[0044] The separation can advantageously be effected by distillation,especially at bottom temperatures of 60 to 220° C. and pressures of 1 to30 bar.

[0045] If the mixture (IV) does not contain ammonia—this also beingunderstood as including such small traces of ammonia as would notadversely affect the following process steps—the mixture (IV) and themixture (IX) are identical.

[0046] In step b), according to the invention, the mixture (IX) isconverted under quantity, pressure and temperature conditions such thatthe diluent (III) and water are in liquid form and exhibit a miscibilitygap, to give a two-phase system consisting of a phase (V) and a phase(VI).

[0047] Preferred quantity, pressure and temperature conditions are suchthat the constituents of the mixture (VII) are in completely liquid formin the phases (V) and (VI), i.e. no solids precipitate out.

[0048] If step a) has been carried out in a homogeneous liquid phase, itis generally possible to separate the mixture (VII) into the two phases(V) and (VI) by choosing an appropriate temperature. A furtherpossibility is to choose appropriate proportions, such as the additionof diluent (III), preferably water.

[0049] According to the invention, the phase (V) and the phase (VI) arethen separated in step c).

[0050] The phase separation can be effected in a manner known per se inapparatuses described for such purposes, for example those known fromUllmann's Encyclopedia of Industrial Chemistry, vol. B3, 5th ed., VCHVerlagsgesellschaft, Weinheim, 1988, pages 6-14 to 6-22.

[0051] The optimum apparatuses and process conditions for the phaseseparation can easily be determined by a few simple preliminaryexperiments.

[0052] In step d), according to the invention, the diluent (III),optionally ammonia and optionally by-products selected from the groupconsisting of low-boiling component [sic] (VIII), high-boiling component[sic] (VII) and unreacted compound (I) are separated from the phase (V)to give a lactam (II).

[0053] In terms of the present invention, low-boiling component [sic](VIII) are understood as meaning compounds boiling below the lactam (II)and high-boiling component [sic] (VII) are understood as meaningcompounds boiling above the lactam (II).

[0054] This work-up can advantageously be effected by fractionaldistillation in one or more, such as 2 or 3, distillation apparatuses.

[0055] Suitable apparatuses are those conventionally used fordistillation, for example the ones described in Kirk-Othmer,Encyclopedia of Chemical Technology, 3rd ed., vol. 7, John Wiley & Sons,New York, 1979, pages 870-881, such as sieve-plate columns, bubble-capcolumns or packed columns.

[0056] Preferably, any ammonia and diluent (III) still present areappropriately separated from the phase (V) first. The high-boilingcomponents (VII), the low-boiling components (VIII) and any unreactedcompound (I) can then be separated from the lactam (II), individually ortogether.

[0057] Advantageously, all or part of the diluent (III) obtained in stepd) can be recycled into step a).

[0058] Advantageously, all or part of any high-boiling components (VIII)[sic] and/or low-boiling components (VII) [sic] obtained in step d) canbe recycled into step a).

[0059] Advantageously, all or part of any unreacted compound (I)obtained in step d) can be recycled into step a).

[0060] The phase (VI) obtained in step c) can advantageously be recycledinto step a).

[0061] Preferably, all or part of the lactam (II) can be separated fromthe phase (VI) to give a mixture (X), and the low-boiling components(VIII) and/or high-boiling components (VII) can optionally be separatedfrom the resulting lactam (II).

[0062] This work-up of the lactam (II) can advantageously be effected byfractional distillation in one or more, such as 2 or 3, distillationapparatuses.

[0063] Suitable apparatuses are those conventionally used fordistillation, for example the ones described in Kirk-Othmer,Encyclopedia of Chemical Technology, 3rd ed., vol. 7, John Wiley & Sons,New York, 1979, pages 870-881, such as sieve-plate columns, bubble-capcolumns or packed columns.

[0064] The high-boiling components (VII) and/or low-boiling components(VIII) can be separated from the lactam (II) individually or together.

[0065] Advantageously, all or part of the high-boiling components (VII)and/or low-boiling components (VIII) can be recycled into step a).

[0066] Particularly preferably, the high-boiling components (VII) areappropriately recleaved before being recycled into step a), as describede.g. in EP-A-793650, EP-A-793651, EP-A-794643 or EP-A-912508. Before thework-up, the lactam (II) obtained from the phase (VI) can also becombined with the crude lactam (II) obtained in step d) and the twoworked up together.

[0067] The phase (X) can advantageously be recycled into step a).

[0068] All or part of the lactam (II) can be separated from the phase(VI) by extraction with a liquid extractant (XI) to give a mixture (XII)containing an extractant (XI) and a lactam (II).

[0069] Suitable extractants (XI) are C₄- to C₉-alkanols such asn-butanol, i-butanol or n-pentanol, preferably aliphatic hydrocarbonssuch as n-hexane, cycloaliphatic hydrocarbons such as cyclopentane orcyclohexane, and especially aromatic hydrocarbons such as benzene,toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, i-propylbenzene ordi-i-propylbenzene, and mixtures of such compounds, for examplepetroleum ethers. The hydrocarbons can carry functional groups such ashalogens, for example chlorine, as in chlorobenzene.

[0070] In particular, the extractants (XI) and the diluents (III) havethe same or a similar composition.

[0071] Thus diluent (III) separated off in step d) can advantageously beused as the extractant (XI).

[0072] The aqueous phase (X) remaining from the extraction canadvantageously be recycled into step a).

[0073] The extractant (XI) and optionally the low-boiling components(VIII), the high-boiling components (VII) and/or unreacted compound (I)can advantageously be separated from the mixture (XII) to give a lactam(II).

[0074] This work-up can advantageously be effected by fractionaldistillation in one or more, such as 2 or 3, distillation apparatuses.

[0075] Suitable apparatuses are those conventionally used fordistillation, for example the ones described in Kirk-Othmer,Encyclopedia of Chemical Technology, 3rd ed., vol. 7, John Wiley & Sons,New York, 1979, pages 870-881, such as sieve-plate columns, bubble-capcolumns or packed columns.

[0076] Preferably, the extractant (XI) is appropriately separated fromthe mixture (XII) first. The high-boiling components, the low-boilingcomponents and any unreacted compound (I) can then be separated from thelactam (II) individually or together.

[0077] Advantageously, all or part of the extractant (XI) obtained inthe work-up can be recycled into step a).

[0078] Advantageously, all or part of any high-boiling components (VII)and/or low-boiling components (VIII) obtained in the work-up can berecycled into step a).

[0079] Advantageously, all or part of any unreacted compound (I)obtained in the work-up can be recycled into step a).

[0080] The mixture (XII) and the phase (V) can advantageously be usedtogether in step d) of the process according to the invention, it beingpossible for them to be combined before or during step d). The lactamsobtained in the process according to the invention can be used in amanner known per se for the preparation of industrially importantpolymers such as polyamides.

We claim:
 1. A process for the preparation of cyclic lactams of formula(II):

in+ which n and m can each have the values 0, 1, 2, 3, 4, 5, 6, 7, 8 and9 and the sum of n+m is at least 3, preferably at least 4, and R¹ and R²are C₁-C₆-alkyl, C₅-C₇-cycloalkyl or C₆-C₁₂-aryl groups, by reacting acompound (I) of the formula

in which R¹, R², m and n are as defined above and R are [sic] nitrile,carboxamide and carboxylic acid groups, with steam in the gas phase,wherein a) the compound (I) is reacted with steam in the gas phase withthe addition, before or after the reaction, of an organic diluent (III)which exhibits a miscibility gap with water under specific quantity,pressure and temperature conditions, to give a mixture (IV) containing alactam (II), b) the mixture (IV) is converted, before or after theseparation of ammonia, under quantity, pressure and temperatureconditions such that the diluent (III) and water are liquid and exhibita miscibility gap, to give a two-phase system consisting of a phase (V)containing a higher proportion of diluent (III) than water, and a phase(VI) containing a higher proportion of water than diluent (III), c) thephase (V) is separated from the phase (VI), and d) the diluent (III) andoptionally by-products selected from the group consisting of low-boilingcomponent [sic], high-boiling component [sic] and unreacted compound (I)are separated from the phase (V) to give a lactam (II).
 2. A process asclaimed in claim 1 wherein the compound (I) used is an aminocarboxylicacid nitrile.
 3. A process as claimed in claim 1 or 2 wherein thecompound (I) used is an aminocarboxylic acid nitrile of the formulaH₂N—(CH₂)_(m)—C≡N in which m is 3, 4, 5 or
 6. 4. A process as claimed inany of claims 1 to 3 wherein the compound (I) used is 6-aminocarboxylic[sic] acid nitrile.
 5. A process as claimed in any of claims 1 to 4wherein step a) is carried out in the presence of a heterogeneouscatalyst.
 6. A process as claimed in claim 5 wherein the heterogeneouscatalyst used is titanium dioxide, aluminum oxide, lanthanum phosphatesor zirconium dioxide.
 7. A process as claimed in any of claims 1 to 6wherein the reaction of step a) is carried out at a temperature rangingfrom 200 to 450° C.
 8. A process as claimed in any of claims 1 to 7wherein the sum of the concentrations of compound (I) and compound (II)is less than 20% by weight, based on the mixture (IV).
 9. A process asclaimed in any of claims 1 to 8 wherein the diluent (III) used is analiphatic, cycloaliphatic or aromatic hydrocarbon.
 10. A process asclaimed in any of claims 1 to 9 wherein the diluent (III) used isethylbenzene, benzene, toluene, o-xylene, m-xylene or p-xylene.
 11. Aprocess as claimed in any of claims 1 to 10 wherein the separation ofammonia from the mixture (IV) is effected before the separation of thephase (V) in step c).
 12. A process as claimed in any of claims 1 to 11wherein the reaction of step a) is carried out in a homogeneous liquidphase.
 13. A process as claimed in any of claims 1 to 12 wherein all orpart of the phase (VI) separated off in step c) is recycled into stepa).
 14. A process as claimed in any of claims 1 to 13 wherein all orpart of the lactam (II) is separated from the phase (VI) separated offin step c) to give a phase (X) containing less lactam (II) than thephase (VI), and by-products selected from the group consisting oflow-boiling component [sic] (VIII) and high-boiling component [sic](VII) are optionally separated from the resulting lactam (II).
 15. Aprocess as claimed in any of claims 1 to 14 wherein all or part of thelactam (II) is separated from the phase (VI) by extraction with anextractant (XI) to give a mixture (XII) containing extractant (XI) andlactam (II), and a phase (X) containing less lactam (II) than the phase(VI).
 16. A process as claimed in claim 14 or 15 wherein all or part ofthe phase (X) is recycled into step a).
 17. A process as claimed in anyof claims 5 to 16 wherein the extractant (XI) and optionally by-productsselected from the group consisting of low-boiling component [sic] (VIII)and high-boiling component [sic] (VII) are separated from the mixture(XII) to give a lactam (II).
 18. A process as claimed in any of claims 5to 17 wherein the mixture (XII) and the phase (V) are used together instep d).
 19. A process as claimed in any of claims 5 to 18 wherein theextractant (XI) and the diluent (III) consist of the same components orhave the same composition.
 20. A process as claimed in any of claims 13to 19 wherein the lactam (II) is separated only from the partial streamof phase (VI) which is not returned to the cyclization step, and all orpart of the remainder of phase (VI) is recycled into step a) withoutrecovering the lactam (II).
 21. A process as claimed in any of claims 1to 20 wherein all or part of the unreacted compound (I) separated off instep d) is recycled into step a).
 22. A process as claimed in any ofclaims 1 to 21 wherein all or part of the high-boiling components (VII)separated off in step d) is recycled into step a).
 23. A process asclaimed in claim 22 wherein the high-boiling components (VII) separatedoff in step d) contain at least 20% by weight of lactam (II).
 24. Aprocess as claimed in any of claims 1 to 23 wherein all or part of thediluent (III) separated off in step d) is recycled into step a).
 25. Aprocess as claimed in any of claims 1 to 24 wherein all or part of thelow-boiling components (VIII) separated off in step d) is recycled intostep a).
 26. A process as claimed in any of claims 15 to 25 wherein thediluent (III) separated off in step d) is used as the extractant (XI).27. A process as claimed in any of claims 1 to 26 wherein all or part ofthe lactam (II) is extracted from the phase (V) or the phase (XII) withwater to give a phase (XIII) impoverished in lactam (II) and a phase(XIV) enriched in lactam (II), and all or part of the phase (XIII) isrecycled into step a).
 28. A process as claimed in any of claims 1 to 27wherein the high-boiling stream (VII) recycled into step a) is combinedwith the recycled stream (VI) or (X) before step a).